API Plan 66B

API Plan 66B

An orifice plug in the drain port minimizes the seal leakage leaving the seal gland and allows for detection of a seal failure by an alarm of the monitoring pressure transmitter.

A  Pressure transmitter
B  Segmented bushing
C  Orifice plug drain

Seal chamber details

1  Flush (F)
2  Pressure transmitter sensing port (PIT)
3  Quench (Q)
4  Drain (D), with orifice plug installed
5  Orifice plug
6  Seal chamber

Example seal: EagleBurgmann H75VA4-S single pusher mechanical seal.

P&ID according to API


3  Quench (Q)
4  Drain (D)

PIT  Pressure transmitter with local indicator


  • Utilizes an orifice plug to limit the amount of leakage leaving the seal gland.
  • Normal leakage continues to flow freely to the drain port.
  • Excessive leakage causes a pressure build-up upstream of the orifice plug which subsequently triggers a pressure alarm by the pressure transmitter.


  • Commonly used on horizontal pipeline pumps.

Remarks, checkpoints

  • It must be ensured that the leakage from the drain port is collected and piped to a leakage recovery system.
  • Drain connection on the gland must be at the bottom-most position (6 o’clock).
  • Media properties to be checked for possible blockage of the orifice plug due to congealing.

Pump media characteristics

  • Low to medium temperature liquids

Location, function

  • Atmospheric side
  • Leakage limiting detection and alarm